[ConstraintSystem] NFC: Replace LLVM_NODISCARD with [[nodiscard]] on newly added method

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -4376,7 +4376,7 @@ class ConstraintSystem {
   /// \param tap the tap expression
   ///
   /// \returns \c true if constraint generation failed, \c false otherwise
-  LLVM_NODISCARD
+  [[nodiscard]]
   bool generateConstraints(TapExpr *tap);
 
   /// Generate constraints for the body of the given function or closure.